The invention relates generally to an endless chain used in an automated transport system and moved under power through a channel track suspended above a rail to propel free travelling trolleys along the rail, and more particularly relates to an improved endless chain capable of being moved under power through such channel tracks at higher speeds and with less friction than other previously known chains.
In automated transport systems of the type which this invention is concerned, the speed at which trolleys can be moved from one position in the system to another is critical. These systems are typically found in garment making plants where workpieces are carried by trolleys between a series of workstations where various work operations are performed on the workpieces or at which locations the workpieces may be moved for storage for transportation later in time. Such a transport system is disclosed in U.S. Pat. No. 4,712,485 issued to Roland P. Nymark on Dec. 15, 1987 and currently assigned to the assignee of the present invention. The aforesaid reference discloses a chain which includes a restricting bracket and a pusher element separated from the associated bracket and carried by the chain to engage with and move a selected one of a series of free travelling trolleys on a rail. However, the chain disclosed in this reference experiences significant problems when the system attempts to function at peak operating levels. Also, these previously known chains are found to be potentially damaging to the workpieces transported in the system.
One problem associated with the chain disclosed in U.S. Pat. No. 4,615,273 is that the wheel journals and the associated axles about which the wheels of the chain rotate, must be lubricated by bathing these journals with a liquid oil film. The liquid oil used to create the lubricating film is extremely offensive in a system where expensive garment material is being moved beneath a traveling chain. In particular, it has been found that the oil lubricating these journals on the chain often drips downwardly from the suspended track and lands on a carried garment piece thereby soiling and damaging such pieces of material during the transporting process. Notwithstanding this point, the oil film used to lubricate this chain is not without its limits. In these previously known chains, the use of an oil film as a lubricant substantially limits the velocity at which the chain can be run. In particular, it was found that at speeds in excess of 35 feet per minute, the oil film supporting the load within the journal does not dissipate frictional heat as fast as it is generated. As such, running the chain at speeds in excess of 35 feet per minute, for example, usually results in localized melting of the component material forming the wheel members in the area of the rotating journals. Consequently, the chains of previously known design could not be moved at speeds in excess of 35 feet per minute. This inability, in turn results in a substantial limitation on the productive capabilities of the transport system as a whole in that time otherwise spent by a worker producing a garment is undesirably spent in travel between work stations on the main rail. Also, these previously known chains create a disturbing amount of noise generated by the insufficiently lubricated wheels moving along the track at high velocities. This noise makes it uncomfortable for workers to perform their designated tasks at their associated work stations. In addition, increased loads brought to hear upon the chain by horizontal forces acting on the chain as it follows the track around a bend, prohibits these previously known chains from moving along a tight radius at high speeds.
Another problem associated with the chains of previously known design is the excessive weight common to each unit length of chain moved under power in the system. Since each link of the chain must be capable of bearing in tension the entire weight of the remaining length of the chain, as well as being capable of supporting whatever additional loads imposed on it by the trolleys being moved, the links often are thick elements formed from heavy materials. This results in the chain being excessively heavy. The excessive weight factor of these chains requires that a significant amount of horsepower be used to move them through the track and around the system. It has been found that for a 400 foot length of a previously known chain, a force of at least 130 pounds is required to set the chain in motion. This force is substantial and is undesirable since it unduly burdens the component parts of the drive means responsible for moving the chain through the system. In particular, the costs in terms of both time and money involved in replacing clutches in the drive motor unit and that involved in the replacement of the motor element itself make it an extremely desirable goal to preserve the drive unit by reducing the loads imposed upon it by the weight of the chain.
Also, it is desirable to minimize the weight of an endless chain used in an automated conveyor transport system such that loads otherwise attributable to the chain's mass may in turn be applied to increase the maximum velocity of the chain or used to carry greater loads at the same velocity. It is known that for a given material, the quantities of pressure and velocity acting on the rotating journals of the wheels are interrelated and, together effect the maximum achievable velocity and/or the maximum load bearing capacity of a given journal bearing. This relationship has sometimes been referred to as the pressure-velocity value of a given material and has been quantified as: EQU PxV=C
where, P represents the pressure in pounds-per-square inch acting on a cross-sectional area of an axle, V represents the surface velocity in feet-per-minute of a journal about an axle surface, and C represents given pressure-velocity constant or value for a particular material. Thus, for a given pressure velocity constant for a particular material, an increase in the maximum attainable velocity may be achieved by decreasing the amount of pressure bearing on the rotating journals of the chain.
There are other factors beside the problems listed above which may cause problems in an automated transport system which robs it of valuable energy and, therefore of valuable needed chain speed. For example, the axles upon which the wheels in the previously known chains rotate are subject to wear, gauling, fretting and corrosion which eventually leads to the seizing up o: the wheels on individual ones of the axles or leads to an outright seizing of the chain in the track.
Accordingly, it is an object of the present invention to provide an improved endless chain for an automated transport system having wheels formed from a self-lubricating material, which wheels being capable of movement along a track in the transport system at greatly enhanced speeds without employing liquid lubricants supplied to the journals upon which the wheels of the chain rotate.
It is yet a further object of the present invention to form the wheels of an endless chain used in an automated transport system from a material having a high pressure-velocity value such that the chain may be moved along a track at velocities in excess of sixty feet per minute while continuing to maintain the same load bearing capability of other, slower moving chains.
A further object of the present invention is to provide a high performance chain of such a construction as to possess one third of the total weight of the previously known chains while obtaining equal if not an enhanced strength in tension.
Still a further object of the present invention is to form the rotating journals of an endless chain used in an automated transport system so as to reduce the amount of pressure acting on these journals for a given load to be moved by the chain.